Automatically detecting the presence of moisture on a surface has many applications. In particular, the ability to detect moisture on an automotive vehicle windshield frees the vehicle operator from the distraction of having to locale controls such as wipers and defoggers when driving conditions change. Windshield moisture can occur as rain, snow, ice, frost, fog, and the like on the windshield outer surface. Moisture may also occur as frost or fog on the windshield inner surface.
Many proposed systems for detecting moisture on a window are based on changes in the reflectivity or transmissivity of the window due to the presence of moisture. Generally, one or more light emitters are aimed at the window inner surface. One or more light sensors is positioned to receive light from the emitters reflected by the window. In one design, emitted light passes through the window when moisture is not present, but is reflected to a light sensor when moisture exists on the inner or outer window surface. In another design, emitted light is coupled into the window at an angle conducive to total internal reflectance when no moisture is present. One or more light sensors are coupled to the window so as to extract light after several internal reflections. The presence of moisture on a window surface degrades the internal reflection, decreasing the amount of light received by the light sensor. In either design, ambient light presents a source of noise that must be compensated for or reduced.
A key element in the design of such moisture detecting systems is the type of light sensor used. This is particularly true in automotive vehicles where the operating environment is severe and cost is a limiting factor. Light sensors must operate within the ranges of temperature, humidity, shock, and vibration experienced within a vehicle passenger compartment. Sensors and support electronics must be inexpensive to allow the cost of automatic equipment, such as windshield wipers and defogger systems, to fall within the range deemed acceptable by an automobile purchaser. The sensor should have sufficient sensitivity across a wide dynamic range. Light transducers within the sensor should have good noise immunity or be compatible with noise compensation electronics within the sensor for sensitivity at low light levels. As a final desirable characteristic, the sensor must be easily integratable into the types of digital control systems commonly found in automotive applications.
One type of light transducer is the cadmium sulfide (CdS) cell. CdS cells are photosensitive resistors exhibiting increasing conductance with increasing light levels. CdS cells have the advantages of being low in cost and having good sensitivity to low light levels. Disadvantages with CdS cells include a high degree of variance between cells, slow response at low light levels, poor environmental stability, and difficulty being assembled by automated electronic manufacturing equipment.
Another type of light transducer used in moisture detecting system is the discrete photodiode configured as a light-dependent current source. Photodiodes have less variance between parts, better environmental stability, and are more easily adapted to automated manufacturing than are CdS cells. However, photodiodes tend to be expensive and produce very low currents at low light levels. These low currents require special amplification techniques to achieve a useful signal, increasing the cost of moisture detection.
Yet another type of light sensor is the phototransistor. The phototransistor functions as a light sensitive amplifier. Light incident on the base generates current which regulates the flow of collector current. Phototransistors are more sensitive than photodiodes but exhibit less stability.
A relatively new type of light sensor incorporates a silicon-based light transducer and conditioning electronics on a single substrate. The light transducer generates charge at a rate proportional to the amount of incident light. This light induced charge is collected over an integration period. The resulting potential indicates the level of light to which the sensor is exposed over the integration period. Light sensors with integral charge collection have many advantages. By varying the integration time, the sensor dynamic range is greatly extended. Also, the ability to incorporate additional electronics on the same substrate as the transducer increases noise immunity and permits the sensor output to be formatted for use by a digital circuit. Component integration additionally reduces the system cost. Silicon devices are more temperature invariant than CdS cells and can be packaged to provide the necessary protection from humidity, shock, and vibration. Types of charge accumulating light transducers include photodiodes and photogate transistors. A variety of charge integrating photodiode devices have been described including those in U.S. Pat. No. 4,916,307 to Nishibe et al.; U.S. Pat. No. 5,214,274 to Yang; U.S. Pat. No. 5,243,215 to Enomoto et al.; U.S. Pat. No. 5,338,691 to Enomoto et al.; and U.S. Pat. No. 5,789,737 to Street. Photogate transistor devices are described in U.S. Pat. No. 5,386,128 to Fossum et al. and U.S. Pat. No. 5,471,515 to Fossum et al. Each of these patents is herein incorporated by reference.
One difficulty with all types of light sensors is the occurrence of operating anomalies at high temperatures. Some devices become extremely nonlinear at high temperatures. Some devices, such as CdS cells, may suffer a permanent change in operating characteristics. Devices may even provide completely false readings such as indicating bright light in low light conditions due to excessive thermal noise. Traditionally, the only way to deal with this problem has been to incorporate a temperature sensor and associated electronics into the moisture detecting system.
What is needed is a moisture detecting system that derives the benefits provided by semiconductor light sensors with integral charge collection. The moisture detecting system should be economical to produce, operate over a wide range of lighting conditions, and be less susceptible to temperature variations.